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Charging by induction occurs when the lower, negatively charged regions of thunderclouds induce a positive charge on the Earth's surface. If the charges become large enough, the resistance of the air is overcome and lightning occurs.

An electroscope is used to detect the presence of electric charges, to determine whether these charges are positive or negative, and to measure and indicate their intensity. This schematic drawing shows the basic parts of the device:

(c) is a glass container, while (d) is a knob that collects electric charges.

Electric charges (either positive or negative) are conducted to the leaves at the bottom via the metal support. Because like charges repel one another, the leaves fly apart. The amount of the charge is calculated by measuring the distance the leaves are forced apart.

A negatively charged rod, A, affects the way charges are distributed in a nearby conductor, B, and a nonconductor, C.

A positive charge is induced on the sides of B and C that are nearest A; a negative charge is induced on the sides of B and C that are farthest from A.

In the conductor, B, the separation of charge involves the entire object because the electrons are free to move. In the nonconductor, C, the separation of charge is limited to the way in which the electrons redistribute themselves within an atom.

This effect is most noticeable if the nonconductor is close to the charged object.

When a charge is stationary or static, it produces forces on objects in regions where it is present, and when it is in motion, it produces magnetic effects.

Electric and magnetic effects are caused by the relative position and movement of positively and negatively charged particles of matter. So far as electrical effects are concerned, these particles are either neutral, positive, or negative.

Electricity is concerned with the positively charged particles, such as protons, that repel one another and the negatively charged particles, such as electrons, that also repel one another. Negative and positive particles, however, attract each other.

This behavior may be summarized as follows: Like charges repel, and unlike charges attract.

Two rods that carry the same kind of charge repel each other. To observe this, obtain two rods that are made of the same kind of material (glass stirring rods, for example). Rub both rods in the same way (with a piece of silk, for example).

If they are made of the same material and have been rubbed in the same way, the rods should carry the same kind of charge. Hang one rod from a thread so that it is free to rotate. Bring the other rod near. The first rod should rotate away from the second, demonstrating that like charges repel.

If the rods had different kinds of charges, the first rod would rotate toward the second, demonstrating that unlike charges attract.